feat: add optional CUDA kernel system (csrc/) + fused GQA decode attention

Structure:
  csrc/               -- .cu sources + build.py registry
  astrai/extension/   -- compiled .so + __init__.py (import dispatcher)
  setup.py            -- CUDAExtension from csrc/build.py REGISTRY

Control: CSRC_KERNELS=true|false env var at install time.
Fallback: astrai.extension.available dict for runtime detection.

csrc/build.py

@@ -0,0 +1,29 @@
+from pathlib import Path
+
+
+def _arch_flag():
+    import torch
+
+    if torch.cuda.is_available():
+        cap = torch.cuda.get_device_capability()
+        ver = f"{cap[0]}{cap[1]}"
+        return f"-gencode=arch=compute_{ver},code=sm_{ver}"
+    return "-gencode=arch=compute_80,code=sm_80"
+
+
+_kernels_dir = Path("csrc/kernels")
+REGISTRY: dict[str, dict] = {}
+
+
+def register(name: str, sources: list[str] | None = None, **kwargs):
+    if sources is None:
+        sources = [str(_kernels_dir / f"{name}.cu")]
+    REGISTRY[name] = {
+        "sources": sources,
+        "nvcc_flags": ["-O3", "--expt-relaxed-constexpr", _arch_flag()],
+        "extra_link_args": kwargs.pop("extra_link_args", []),
+        **kwargs,
+    }
+
+
+register("gqa_decode_attn")